Don't Divvy Up The Dewormer

Q. When planning a deworming program, how do you prevent new parasite eggs from hatching? You deworm and kill the worms, but then more eggs hatch and reinfect. Is it beneficial to give, say, 5 grams of dewormer per day for four days or every other day to kill newly hatched worms? Or is it best to give it all at once? — Sherri Knippling, via email

A. Pastured horses are exposed to a nearly continuous cycle of reinfection with cyathostomins (small strongyles). Effective deworming provides a temporary respite from adult worm populations (which are relatively harmless), but the same pool of pasture larvae that infected your grazers yesterday is still out there in the grass waiting to do it again tomorrow. Deworming disrupts parasite egg production temporarily because the anthelmintic treatment kills the egg-laying adult female worms. No hen, no eggs.

Deworming might even halt egg production for up to several weeks after administration, especially when using ivermectin or moxidectin. However, any benefits from reducing the infective potential of the environment will not occur immediately but, rather, in weeks or months. Depending on the season and local geoclimatic conditions, the cyathostomin larvae currently threatening your horses could be as young as one week or as old as six months, and they can survive on pastures for a fairly long time. So, the overall risk of reinfection doesn’t stop immediately with deworming. However, because worm eggs turn into infective larvae, the risk of infection should be diminished, at least theoretically, thanks to deworming.

So, this begs the question: If I deworm the entire herd more frequently and prevent my horses from passing any eggs into the environment, won’t that eventually minimize pasture contamination and, thus, the risk of reinfection? Indeed it can, and that was the logic behind the traditional deworming strategy known as interval treatment (roughly, treating every two months all year long). However, the very practice of deworming horses frequently, without bothering to determine which members of the herd were the worst offenders (i.e., had the highest egg counts) or confirming that the worms were still susceptible to the anthelmintics being used, is largely responsible for the current high prevalence of anthelmintic resistance to most of the dewormers on the market. Interval treatment has now been thoroughly discredited, and you should contact your veterinarian to learn more about selective anthelmintic treatment (SAT) programs. These can accomplish effective parasite control with fewer treatments and less selection for anthelmintic resistance.

Selection for anthelmintic resistance occurs within portions of worm population that are exposed to the drug during deworming. Infective larvae on pasture and any worms within horses that go untreated are, by definition, not “selected.” With selection (i.e., deworming), worms that survived the treatment probably did so because they carry genes imparting resistance to the anthelmintic. These resistant worms then have the temporary advantage of being able to reproduce in the absence of competition, thereby increasing the proportion of resistant genes in the entire worm population. We call unexposed populations “refugia,” which is essentially a reservoir of genes susceptible to the drug. The basic strategy is to deworm as few members of your herd as feasible to maintain health and to deworm them as infrequently as possible. Think about it: If we never dewormed, resistance would never develop.

It’s never a good idea to use any veterinary drug in a manner that is inconsistent with the explicit label directions. So, dividing a treatment into multiple subtherapeutic doses might not only be ineffective but also theoretically select for dewormer resistance.

The main circumstance in which egg passage immediately after deworming might be significant is in the case of new horse arrivals to the farm. Any new horse from a different premises could be carrying worm populations that do not exist in your herd, and this is no time to embrace diversity. Horses can still pass viable eggs for a few days after deworming, so in the case of new introductions, it’s wise to keep them stabled for four to five days after anthelmintic treatment (and perhaps up to 14 days to demonstrate that the dosing was effective) before turning them out to communal pastures. Compost their feces rather than spreading them on pasture to avoid introducing new pathogens to which the resident herd has no immunity.